File:The Quarterly journal of the Geological Society of London (12645076783).jpg

1848.. NICOL ON RECENT FORMATIONS NEAR EDINBURGH. 21
tons in weight, and generally derived from trap, sandstone, or lime-
stone rocks, like those composing the coal-field on which it rests.
Fig. 2.
Some of these boulders however consist of granite, mica-slate or
other primary strata, and must consequently have been carried a
greater distance, as none of these rocks are found nearer than from
forty-five to fifty miles, and granite in any quantity only at seventy
miles' distance. These boulders are generally rounded and water-
worn, but some on the contrary are angular. They are found in
every part of the mass of blue clay, but, as it seemed to me, in more
abundance in certain portions, and apparently arranged in horizontal
lines.
These facts appear to prove that the deposition of this boulder
clay or till was gradual, — the effect of long-continued and variable
agents ; and not of a sudden rush of water, or debacle, as has been
imagined. The whole phsenomena seem more consistent with the
supposition that the clay was formed by the continuous action of the
sea on the various strata of the subjacent coal-field, than with any
other theory. The blue clay forming the great bulk of the till may
be regarded as merely the decomposed shales of the coal formation,
and the sands as comminuted sandstones : even the relative position
of the deposits, with the blue clay below, and a browner and more sandy
clay resting upon it, as seen in the sections, favours this opinion. The
soft shales when exposed to the action of the waves would be wasted
away before the harder sandstones and trap rocks, and the deposit
formed from their destruction would consequently occupy a lower
position. The boulders may have been brought to the place where
we now find them by ice, or entangled in the roots of floating trees,
or in any other mode now in action for the transport of rock masses.
Though mixed up irregularly with the mass of clay, it is by no means
necessary that they should have been always transported along with it
or by the same agent. Were a number of boulders at the present
day dropped on a mass of soft semifluid clay at the bottom of the sea,
they would not remain on the surface, but sink in it to various depths,
and thus appear to have been deposited by the same agents, when in
reality they were deposited by wholly different causes. Neither does